1. Field of the Invention
This invention relates to a catalyst and a purifying process for purifying nitrogen oxides (NOx) contained in exhaust gases exhausted from internal combustion engines of automobiles and the like, nitric acid production plants, and so forth.
2. Related Art Statement
The nitrogen oxides (NOx) and so on have been contained in the exhaust gases exhausted from internal combustion engines of automobiles and the like, nitric acid production plants, and so forth. There have been many investigations on the purification of the nitrogen oxides contained in the exhaust gases.
So far, reducing catalysts such as noble metals and base metals have been mainly employed to purify the nitrogen oxides contained in a reducing gas. However, few catalysts have been known which purify the nitrogen oxides contained in an oxidizing gas.
According to pages 364 to 365 of the preprints of the sixth catalyst forum in Japan, 1987, there is a report notifying that metallic silicates are useful as catalysts for acid catalyst reaction, ammoxidation, nitrogen monoxide (NO) decomposition and carbon monoxide (CO) oxidation reaction. The metallic silicates are obtained by substituting aluminum of pentasyl type zeolite "ZSM-5" with various metals after synthesizing the zeolite. For example, the report discusses the oxidation reaction catalyzed by the silicate catalyst containing copper, in which propylene is oxidized to acrolein in the oxidation reaction. One of such catalysts is an aluminosilicate (zeolite) with copper loaded by an ion-exchange method. However, the report does not disclose the purification of nitrogen oxides (NOx) and the like contained in the exhaust gases. Though the copper silicate described in the report has an activity as an oxidizing catalyst, it contains less amount of copper and has less durability as an exhaust gas purifying catalyst.